Implementing FUSE exports required no changes to the storage daemon, so
we forgot to document them there.  Considering that both NBD and
vhost-user-blk exports are documented in its man page (and NBD exports
in its --help text), we should probably do the same for FUSE.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210217115844.62661-1-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>